The present invention generally relates to a microwave heating method and apparatus for effecting a vacuum cooking operation (sous vide) with high frequency heating.
The vacuum cooking operation cooks vacuum packed foods at a constant temperature between approximately 55.degree. C. and approximately 95.degree. C. using either boiling water or steam. It has following advantages. (A) The heat conduction is superior because of the vacuum. A uniform heating can be effected at a specific temperature which ensures the most delicious taste with respect to foods. (B) The permeation of seasonings is superior because of the vacuum. The seasoning can be effected using only small amounts of sugar and salt, thus being desirable from the health standpoint. (C) Food is vacuum packed so that the flavor is not diminished. (D) Food is heated at low temperatures so that lines, fibers and so on remain soft without becoming hardened. (E) The yield is considerably higher, because food is cooked at temperatures where water division of protein is not caused. (F) Foods can be preserved for approximately one week in cold storage so that the mass supply of foods for banquets at a hotel can be conveniently provided. Vacuum cooking was s invented in France and has spread quickly.
The humid environment of a kitchen where hot water of 60.degree. C. through 95.degree. C. is kept is not favorable as judged easily from the humid environment within the bath chamber in which the hot water temperature is 42.degree. C. through 43.degree. C. The environment has a risk of being dangerous enough to cause burns. Therefore, improvements in the environment are strongly desired. A reduction in the high fuel expenditure needed to maintain the high temperatures is also desired. Similar problems arise in the case of steam ovens.
As a solution to the above problems, the use of a high frequency heating apparatus such as electronic range or the like has been considered. It is extremely difficult to effect a solution using a conventional apparatus, because the final temperature accuracy demanded during the vacuum cooking operation is approximately 1.degree. C. Although various methods are used in France, the results are said to be failures. The final temperature accuracy of foods in the conventional apparatus will be approximately 20.degree. C. at its highest.
The uniform heating methods used by the conventional apparatus can be chiefly classified into four methods.
Firstly, one tries to make the electromagnetic wave distribution uniform. Various ideas represented by stirrer blades or turntables have been disclosed. The trials are too numerous to mention.
Secondly, a method which is used widely in the conventional cooking operation using fire is used as is. Wave concentration onto one portion is prevented or a high temperature portion or an excessively heated portion are cooled so as to make them uniform. Aluminum foil is used as the wave concentration prevention so as to effect a wave shielding operation. Defrosting the frozen foods in cold air is introduced as a cooling method in U.S. Pat. No. 3,536,129.
Thirdly, what is generally called weight-defrosting or weight-cooking is widely used. A heating operation is effected with the irradiation power and the irradiation time of optimum waves being set in accordance with the food weight; foods are left without the application of microwaves for an optimum standing time and the temperature becomes uniform due to the thermal conduction of the food interior. The U.S. Pat. No. 4,453,066 is one example of such a method.
Fourthly, the temperature of the food is detected so as to control the application of microwaves. There are patents such as the U.S. Pat. No. 3,634,652 (foods are retained at a given temperature or lower using a sensor), and the U.S. Pat. No. 4,785,824 (optical fiber thermometer is used) in addition to the U.S. Pat. No. 2,657,580 (multirange thermometer). Japanese Laid-Open Patent Publication No. 52-17237 discloses a plurality of locations in food in which the temperature is detected; the microwave output is lowered at a time when the set temperature has been reached at one location, and the heating is completed at a time when another has reached the set temperature.
Japanese Laid-Open Patent Publication No, 54-7641 discloses a method of estimating the internal temperature from the food surface temperature; the microwave irradiation is stopped when the surface temperature has reached 5.degree. C. during the defrosting of the frozen food; microwaves are applied again at a time when the surface temperature is as low as 0.degree. C., and differentiation values in time change from 5.degree. C. to 0.degree. C. are detected).
But it is impossible to have the temperature of each portion of the food be within several degrees C or lower, although it is not said that a 1.degree. C. or lower difference is necessary, with respect to the desired final temperature at the completion of the heating by these methods.
If, for example, the temperatures of each portion of the food can be measured correctly heating can be easily realized by an advanced controlling method using computers in an estimation controlling operation or the like. However, only one portion becomes 65.degree. C. if a heating operation is effected to, for example, 65.degree. C., or the other portion remains cold without being heated (described later in detail).
Although relatively good results are obtained even in a method of gradually reducing the application of microwaves to be used in defrosting operation, the latent heat of 80 calories at 0.degree. C. becoming a buffer during the defrosting operation. The difference between the desired temperature and the actual final temperature is large and also, the temperature difference between different portions of the food is also large.
In the vacuum cooking operation, a heating operation is effected with, for example, a final temperature of 65.degree. C. as a target, and variations of +10.degree. C. or -10.degree. C. occur, and thus the final temperature is between 55.degree. C. and 75.degree. C.